Polyurethane coatings are by now well known. For example, polyurethane varnishes exhibit excellent resistance to water and solvents, and are widely used in finishing articles of wood, metal, and plastic. However, such varnishes are solvent-borne, and the introduction of solvent into the air by evaporation is environmentally undesirable. Many states have enacted legislation limiting VOC (volatile organic compounds) emissions in industrial processes, and this ban has had an effect even on home use of such products. As a result, there is a great interest in solvent-free compositions or aqueous compositions where only water is evolved upon drying.
Aqueous polyurethane lattices have been developed which offer excellent film forming capabilities. Such lattices are typically dispersions of polyurethane/ureas in water. Presently available polyurethane dispersions offer relatively long shelf life, but the range of hardness and flexibility is limited. For example, in textile applications, solvent borne polyurethane lacquers are still predominant, as aqueous dispersions have not provided the degree of softness and elasticity required to prepare textiles having aesthetic hand and drape.
Polyurethane lattices are generally prepared by the reaction of excess di- or polyisocyanate with a hydroxyl-functional polyol in the presence of reactive compounds capable of providing a degree of hydrophilic character to the prepolymer thus formed. The reactive compound may contain cationic or anionic groups or non nonionic hydrophilic groups such as polyoxyethylene groups. The prepolymer is then dispersed in water, either neat or dissolved in solvent. This dispersion is then reacted in situ with a chain extender, generally a diamine, triamine, or the like, to form a stable, aqueous polyurethane/urea latex. If solvent is utilized, it is then stripped off, leaving an aqueous dispersion. The charged or hydrophilic groups in the prepolymer are necessary in order to effect long term stability to the dispersion, which otherwise might settle or coagulate.
Among the hydroxyl-functional polyols utilized to form the isocyanate-terminated polyurethane prepolymer are polyester polyols such as polybutylene adipate and polycaprolactone diols, polyoxytetramethylene ether glycols (PTMEG), polyoxyethylene diols and polyoxypropylene diols. If a minor degree of crosslinking is desired, a trifunctional polyol such as a trimethylol propane or glycerine initiated polyoxyalkylene polyol may be added. Increased hardness may be achieved through addition of a low molecular weight glycol, for example ethylene glycol or neopentyl glycol.
Polyoxytetrmethylene ether glycols (PTMEG) are preferred in some applications, as the cured films possess a desirable combination of properties, for example excellent hydrolyric stability and microbial resistance, in comparison to films prepared from similar molecular weight polyesters. PTMEG is, however, a premium cost raw material. Moreover, films formed even from PTMEG-based lattices are deficient in flexibility and elongation for many applications. Polyurethane lattices have also been prepared from conventional polyoxypropylene diol-based prepolymers. However, films prepared from such prepolymers have inferior tensile strength and elongation, and have a tendency to be tacky in comparison to their PTMEG analogs. Applications of such polyurethane dispersions are therefore limited to uses where cost rather than performance is the driving factor. Thus, it would be desirable to be able to provide low cost polyurethane dispersions with flexibility, elongation, and other physical properties which equal or exceed those of lattices based on PTMEG derived prepolymers.